1) Field of the Invention
The present invention relates to a technology for fixing a toner image on a recording medium by holding the recording medium between a nip section formed between two bodies that move relatively to each other.
2) Description of the Related Art
In an electrophotographic or an electrostatic image forming apparatus, normally an electrostatic latent image is formed on an image carrier such as a photosensitive drum or a photosensitive belt according to image information. A toner image is formed by allowing toner, which is charged, to adhere to the electrostatic latent image. The toner image is then fixed on a recording medium by the application of heat or pressure.
A roller fixing method in which a pair of rollers which are in contact with each other as fixing members has been known as a method of fixing a toner image on the recording medium. In the roller fixing method, one of the rollers is used as a heating roller and the other roller is used as a pressurizing roller. A fixing nip that holds and carries the recording medium is formed at a position where the two rollers come in contact with each other. Heating and pressurizing is performed while the recording medium passes through the fixing nip and toner that is not fixed is melted and pressed on the recording medium, thereby getting the toner fixed on the recording medium.
Apart from the roller fixing method, a belt fixing method in which any one or both of the heating roller and the pressurizing roller are substituted by a belt is know (refer to Japanese Patent Application Laid-open Publication No. H11-282307).
However, in these fixing methods, due to the direct contact of the unfixed toner carried on the recording medium with a fixing member such as the roller and the belt, a hot offset tends to occur easily. The hot offset is a phenomenon in which a part of the unfixed toner is reversed back to the fixing member and adhered to it while fixing. When the temperature of the fixing member is high, there is a reduction in cohesive force of the melted toner, thereby leading to easy occurrence of the hot offset.
On the other hand, it is desirable to reduce power consumption to save energy. The following are methods that allow reduction in the power consumption. (1) Stop power supply to the fixing unit when the fixing unit is not in use, and (2) Perform fixing at a low temperature.
Applicants of the present invention have proposed a toner that can be fixed at a low temperature in Japanese Patent Application Laid-open Publication No. 2002-162773. Moreover, a structure that enables to reduce an amount of electricity required during a waiting time starting from the passing of electricity until image forming (warming up time of the unit) to minimum extent has been proposed in Japanese Patent Application Laid-open Publication No. 2003-156959.
The energy conservation has become an important issue since ecology has been drawing more and more attention in recent years. Therefore, an accomplishment of the energy conservation has been sought after.
Further, a technology of fixing an image on a large amount of recording media at a speed higher than that achieved so far, without increasing the amount of electric power used by the fixing unit, has been sought after.